Excursions in the internal temperature in rechargeable lithium-ion cells may occur rapidly, within milliseconds to tens of seconds. Moreover, a relatively modest but rapid elevation of internal temperature may result in damage to a solid electrolyte interface/interphase (SEI) layer, electrolyte degradation, and/or chemical decomposition within the cell. While initial damage may be heat-induced, the initial damage leads to an exothermic reaction, which may result in an auto-catalytic process known as thermal runaway. Consequently, a sudden heat excursion may lead to initial damage in the form of a beak-down or rupture of the SIE layer. This may place an electrolyte in contact with a carbon anode, which may result in a reaction that releases additional heat and leads to thermal runaway.
Prior studies indicate that a nominally protective SEI layer may be damaged at temperatures as low as 70° C. Moreover, the time scale associated with thermal runaway is short, depending on a state-of-charge (SoC). At an ambient temperature of approximately 25° C. and 100% SoC, thermal runaway may occur within a few seconds, whereas at less than 50% SoC, thermal runaway may occur over tens of seconds.
Conventional surface-mounted temperature sensors do not adequately track internal temperature changes, regardless of a sampling rate used to digitize sensor output. This is because surface mounted temperature is a low-pass-filtered reflection of internal temperature. In other words, surface-mounted temperatures sensors do not reflect fast changes in internal temperature, such as changes over less than a few seconds. Conventional surface-mounted temperature sensors are thus inadequate to detect or predict thermal runaway.